This invention generally relates to a loudspeaker having a diaphragm comprised of a resin film and a voice coil in the form of a conductor pattern which is formed on the resin film by etching or vapor deposition of a thin film or foil and more particularly, it is concerned with a loudspeaker structure having a diaphragm formed with a plurality of multi-layer voice coils, and a loudspeaker system having the loudspeaker structure and a dividing network associated therewith as its interface.
One type of loudspeaker using a resin film as a diaphragm and another type having a plurality of separate voice coils are known as will be explained later with reference to conventional examples. These types of loudspeaker have inherent advantages but are disadvantageous in that their working band, i.e., reproduction band is restricted.
A background underlying the present invention will first be described by way of example.
In recent years, a heat-proof synthetic resin film (hereinafter simply referred to as a film) has been available, and a loudspeaker has been manufactured which has a thin diaphragm or membrane prepared by bonding together the film and an aluminum foil, for example, and etching the aluminum foil to form a conductor pattern of voice coil. Advantageously, this type of loudspeaker has a light vibration member and can afford to exhibit a flat electrical impedance characteristic, thereby greatly improving transient characteristics as compared to conventional cone type and dome type loudspeakers.
On the contrary, as described in an article "Tweeter Using New Structure and New Material for Diaphragm", Audio Eng. Soc., Vol. 29, No. 10, '81 Oct, the lightness of the vibration member degrades flatness of sound pressure/frequency characteristics in such a manner that as the frequency increases, the sound pressure level increases, causing a disadvantage of occurrence of a bounce h as shown in FIG. 2.
The occurrence of bounce h is due to radiation impedance characteristics of the loudspeaker, especially, an X component of a radiation impedance Z.sub.r which varies with frequency as shown in FIG. 3.
The radiation impedance Z.sub.r is given by ##EQU1## J.sub.1 is a Bessel function, S.sub.1 is a Struve function, a is an effective radius of the diaphragm, Zo is a characteristic impedance which is Zo=.rho..sub.o C where .rho..sub.o represents the density of air and C sound velocity, and k a wave number. The real term R.sub.r is called a radiation resistance and the imaginary term X.sub.r is called a radiation reactance and within a frequency band represented by ka&lt;1, there terms approximate, ##EQU2## where k denotes the wave number which is .omega./C or 2.pi./.lambda., .lambda. a wavelength, .omega. an angular frequency which is 2.pi.f, and f a frequency.
In spite of the fact that, in many applications, the loudspeaker using the film is rectangular, the loudspeaker described herein is assumed to be a piston disc in an infinite baffle for simplicity of explanation.
Assuming that X.sub.r is represented by ##EQU3## M.sub.a is representative of a mass which is independent of frequency. This mass M.sub.a is a mass added to one surface of the diaphragm and called an additional mass of air, which represents an amount of inertia to which the diaphragm is subject when it causes air to vibrate. For a loudspeaker in an infinite baffle, the amount of inertia is doubled. The relation indicated by Equation (7) is substantially valid for a lower frequency band represented by ka&lt;1 but for a higher frequency band of ka&gt;1, the radiation reactance X.sub.r gradually decreases as shown in FIG. 3, followed by a decrease in M.sub.a.
On the other hand, for the band of ka&lt;1 within a mass controllable region satisfying f&gt;f.sub.o, f.sub.o being a minimum resonance frequency, an output sound pressure level (SPL) is determined by, ##EQU4## where C.sub.o : constant
B: magnetic flux density at magnetic gap PA1 l: length of voice coil PA1 M.sub.o : effective mass of vibration member (M.sub.o =M.sub.d +M.sub.v +M.sub.a + . . . ) PA1 Z.sub.s : electrical impedance of voice coil.
In M.sub.o, M.sub.d designates a mass of the diaphragm, M.sub.v a mass of the voice coil and M.sub.a an additional mass of air. For the lower frequency band of ka&lt;1, the additional mass of air M.sub.a is substantially constant as represented by Equation (8) but for the higher frequency band of ka&gt;1, the additional mass of air gradually decreases as described previously in accordance with the following equation: ##EQU5##
For a high frequency band as represented by ka&gt;5, EQU R.sub.r .apprxeq..pi.a.sup.2 Z.sub.o ( 11) EQU X.sub.r .apprxeq.0 (12)
are held and M.sub.a .apprxeq.0 results. This explains that the bounce h, leading to degraded tone quality which gives uncomfortable feeling to hearing, occurs in the output sound pressure/frequency characteristics as shown in FIG. 2. From Equation (9), the bounce h is indicated by ##EQU6##
When a diaphragm is designed to have a 1.3 cm.times.5 cm rectangular configuration, an additional mass of air 2M.sub.a of about 15 mg and a mass M.sub.d of 7 mg, bounce characteristics of a loudspeaker using this diaphragm are calculated for a voice coil having a mass M.sub.v of 21 mg and another voice coil having a mass M.sub.v of 7 mg. The smaller the mass M.sub.v of voice coil, the greater the bounce h becomes. This means therefore that the smaller the mass M.sub.o of vibration member, the greater the bounce h becomes. The article previously described proposes an expedient for elimination of the bounce h, according to which a horn 2 is disposed in front of a diaphragm 1 as shown in FIG. 4 to thereby flatten the sound pressure/frequency characteristics. However, since M.sub.a is proportional to the cube of effective radius a of the diaphragm, the bounce h increases as the diaphragm increases in size.
Take, for instance, an all band type loudspeaker of a diameter of 30 cm which is dimensioned such that a=12 cm, M.sub.a =11 g, M.sub.d =0.8 g, film thickness t=12 .mu.m, M.sub.v =2.6 g, aluminum foil thickness=20 .mu.m and M.sub.o .apprxeq.14.4 g. In this loudspeaker, a bounce h occurs which amounts up to about 12 dB as shown in FIG. 5. If an attempt is made to eliminate this amount of bounce with the expedient of the aforementioned article wherein the horn is disposed in front of the diaphragm to flatten the sound pressure/frequency characteristics, then the horn mouth length of more than 1.5 m longitudinal length which is practically unacceptable.
For the reasons set forth previously, the diaphragm is mainly used for tweeters.
It is particularly important to note that the additional mass of air M.sub.a is in proportion to the cube of the effective radius a of the diaphragm with the result that as the diaphragm increases in size, the bounce h becomes large correspondingly.
Incidentally, for the high frequency band of ka&gt;5 described previously, R.sub.r and X.sub.r are given by Equations (11) and (12), indicating that directivity becomes so sharp that radiation of sound is confined in the front of the diaphragm and hence approximates a plane wave.
A directivity characteristic is determined by the following equation: ##EQU7## where R.sub..theta. is a ratio between an on-axis sound pressure and a sound pressure in a direction making an angle .theta. to the axis. Thus, the angle between the axis of the diaphragm and the projection of the line joining the center of a measuring point and the origin of the axis is .theta.. There are illustrated in FIG. 6 a sound pressure/frequency characteristic for .theta.=30.degree. as represented by a dashed curve (b) and that for .theta.=60.degree. as represented by a dashed and dotted curve (c).
As will be seen from FIG. 6, with a larger diaphragm, sharpness of the directivity becomes eminent at a lower frequency.
Japanese Patent Unexamined Publication No. 55-25265, published on Feb. 22, 1980, discloses a loudspeaker having a plurality of voice coils wound on a single bobbin as shown in FIG. 7. The loudspeaker generally designated at 3 in FIG. 7 comprises a voice coil 4 for reproduction over all band or range, and a voice coil 6 cooperative with a low-pass filter 5 for reproduction of signals at 200 Hz or less. This loudspeaker provides sound pressure/frequency characteristics as graphically shown in FIG. 8 wherein reproduction pursuant to a characteristic (d) is obtained by the voice coil 4, reproduction pursuant to a characteristic (e) is obtained by the voice coil 6, and reproduction pursuant to a composite characteristic (f) which is emphasized for 200 Hz or less is obtained by both the voice coils 4 and 6.
This loudspeaker 3 also has an electrical impedance pursuant to a characteristic (g) wherein the electrical impedance Z.sub.s falls below a predetermined value, for example, 8 .OMEGA. as the frequency decreases below 200 Hz because the voice coils 4 and 6 are driven in parallel. Further, an inductance attributable to the winding of the voice coils causes the electrical impedance Z.sub.s to increase as the frequency increases, bringing about a snaky electrical impedance characteristic as represented by the characteristic (g). Because of this electrical impedance characteristic, a power amplifier for driving the loudspeaker tends to suffer from unstable operations.